Determination of rock boundaries below drill bit are useful in helping drill planning, such as deciding the optimum location of casing shoes. Rock acoustic velocity is an important parameter in seismic data processing and interpretation. Conventionally, rock boundaries and rock acoustic velocity are determined with a technique, known as vertical seismic profiling (VSP), using an external seismic source and seismic listening devices (geophones) attached to the casing wall. The installation of geophones and the VSP measurement can require drilling suspension for significant periods of time. Long stops or breaks of drilling can significantly increase the cost of the drilling operation in addition to the cost of the actual profiling operation.
Some systems that attempt to make the measurement without drilling breaks use the drill bit sound as the seismic source and an array of geophones that arranged on the surface to record the sound transmitted from the drill bit. Unlike seismic source used in seismic methods, the drilling bit sound is not impulsive, rather it is continuous. As such, the information recorded by the geophones is a mixture of drill sound transmitted directly from the source (drill bit) and the sounds reflected from rock boundaries. It would be useful to find a method that separates the direct drill sound and the reflected ones in order to determine the location of the rock boundaries below drill bit. Furthermore, the drill sound significantly attenuates within rock formations with distance, affecting the quality of the transmission to the receivers placed on the ground surface.